He declared victory a day earlier, following preliminary exit polls that suggested he had secured a majority of the vote.

His closest challenger, Yulia Tymoshenko, a former Ukrainian prime minister and the leader of the Batkivshchyna party, conceded the election after exit polls showed her with 13% of the vote, which matched Monday's early official tally.

Poroshenko, a candy tycoon known as the "Chocolate King," is also a seasoned politician known for his pro-European Union views.

At a news conference in Kiev, he reiterated that European integration would be his priority.

He added that in Sunday's vote, the President and the whole of Ukraine had changed.

He said Russia needs to participate in bringing peace to eastern Ukraine.

Voters were picking a successor to the ousted pro-Moscow President, Viktor Yanukovych, in a country torn apart by Russia's takeover of Ukraine's Crimea peninsula and a bloody conflict involving pro-Russia factions.

The unrest has centered in the Donetsk and Luhansk regions, where separatists have claimed independence after a disputed referendum this month -- and many there did not get to cast ballots Sunday.

Reports from the region indicated that perhaps 75% of polling stations were closed, according to regional officials.

A large separatist rally was held in a central Donetsk city square around lunchtime Sunday.

The protesters, who chanted pro-Russia slogans as they were addressed by separatist leaders, were joined by a substantial number of militants on trucks, some firing guns into the air.

On the back of some of the trucks were armed men who appeared to be Chechen.

Two told a CNN team they were from the Chechen capital, Grozny, and one indicated that he was formerly a policemen in Chechnya and was in Donetsk to serve the Russian Federation.

The men, who as Chechens are Russian citizens, said they were there as "volunteers."

But if their accounts were true, their presence in Donetsk would appear to indicate some kind of acquiescence by the Russian government at the least.

Residents of Ukraine's southeastern city of Mariupol saw new billboards on the streets Sunday urging them not to cast their ballots.

The billboards were not at those locations the night before, residents said.

Also in Mariupol, people talked on social media about being asked by local Russia supporters to boycott the election.

The city is one of several where deadly clashes have erupted in recent weeks.

The self-declared mayor of rebel stronghold Slavyansk, Vyacheslav Ponomaryov, has said that anyone who tried to vote there would be arrested.

Also, an Italian journalist was killed Saturday near the flashpoint town, the Italian Foreign Ministry announced Sunday.

The man, named as Andrea Rocchelli, was killed along with a Russian citizen, the ministry said.

Reports suggested there had been mortar fire in the Slavyansk area.International monitors

Besides the presidential race, candidates were also running in municipal elections in some cities.

The Organization for Security and Co-operation in Europe deployed 900 observers for the election, the largest such mission in its history.

At a news conference Monday, OSCE Parliamentary President Joao Soares said the election was fair and represented the will of the Ukrainian people, despite major problems in two eastern provinces.

He said the election sent a message to the pro-Russia militants:

"You cannot win by breaking the rules."

Oleksandr Turchynov, Ukraine's acting President, called the polls open and transparent.

"The voting was free, without artificial restrictions and administrative pressure," Turchynov said in a statement.

U.S. President Barack Obama congratulated Ukrainians for casting their ballots Sunday and criticized Russia-backed separatists, whom he accused of trying to block voting.

"Despite provocations and violence, millions of Ukrainians went to the polls throughout the country, and even in parts of eastern Ukraine, where Russian-backed separatist groups sought to disenfranchise entire regions, some courageous Ukrainians still were able to cast their ballots," he said in a written statement.

"We commend the resolve of all those who participated, as well as the efforts of the Ukrainian government to conduct these elections in the face of those threats."

But Russia's Lavrov offered a different view Monday, saying that not all candidates were given an equal opportunity to present themselves to the nation and campaign properly.

He said many even faced threats to their lives, and others were assaulted.